NEW TWO-STROKE MARINE DIESEL ENGINES FROM WÄRTSILÄ

Wärtsilä has developed a new generation of small marine diesel engines with the designations RT-flex35 and

RT-flex40 with cylinder bores of 35 cm and 40 cm. The engines are equipped exclusively with an integrated

electronic control system, and for the first time in a low-speed two-stroke engine, a common rail fuel injection

system has been adopted from the medium-speed four-stroke engine. The bore designs are based on a joint

concept with Mitsubishi Heavy Industries and were developed in close cooperation. The acceptance test of the

first engine, in accordance with the order of a six-cylinder engine, is planned for november 2011.

cover story LARgE EngInES

10

Motivation

In 2008, Wärtsilä decided to reinvigorate the small two-stroke marine diesel seg-ment by bringing out two newly devel-oped engines. The last new addition to the segment within the own company was the Sulzer RTA38 engine in the mid-eighties. In Asia in particular, there is a strong market for direct propeller-driven low-speed crosshead engines. The ben-efits are that they are cheap to produce, highly available thanks to local licensed production, simple to maintain and have low running costs [1]. Up to a bore size of around 40 cm, these engines are used in small and medium-sized commercial vessels, such as handysize bulk carriers and product tankers, general cargo ves-sels, container feeder ships, and small LPG carriers.

With Mitsubishi Heavy Industries Ltd. (MHI) in Kobe, Japan as a development partner, it was possible to build on the good working relationship from a previous devel-opment project, and this proved worth-while with respect to Mitsubishi’s exten-sive experience with the small UE range. MHI are deriving their own 35LSE and 40LSE UEC engines with mechanical injec-tion from the RT-flex35 and RT-flex40 [2].

DevelopMent objectives

The development objectives were hence defined: : low product life-cycle costs : low emission level : use of established technologies.Reliability is always the top priority for all low-speed two-stroke engines used for marine power. For licensees, the fact that the engine is simple and economical to produce is also a key factor. From the point of view of the ship owner, operators and licensees, there is a need to minimize costs incurred throughout the life cycle of the engine, although the priority of this require-ment can vary.

The RT-flex35 is the first new two-stroke engine from Wärtsilä which complies with the IMO Tier II emissions level as of its market launch. New technologies include a new design principle for the engine struc-ture, a new injection and cylinder lubrica-tion system, a turbocharger at the aft end of the engine and a simplified piston cool-ing arrangement. This means that simula-

tion and test validation play a particularly important role, as even the first two-stroke engines in the new series will be serving a time of 25 years and longer. In order to achieve the global objectives of lower costs and emissions at the same time as high levels of reliability, a new process was used in the development of the engines which is briefly described later on.

Main Design Features

❶ shows the speed and engine perfor-mance figures for new engines in the two-stroke engine portfolio from Wärtsilä. In ❷ the main data for the new engines is listed in direct comparison to other engines. The specific figures show that the current two-stroke marine diesel engines are high-tech products. The basic features of the new engines are similar to those of typical large modern low-speed crosshead diesel engines, ❸.

engine structure

The engine structure is a welded construc-tion with bedplate and column. The bed-plate supports the crankshaft contained in it by means of a welded cast steel girder. The column, which is bolted to the bed-plate and cylinder block through tie rods, incorporates the guide rails for the cross-head, through which the lateral forces from the powertrain are fed into the structure.

powertrain

The crankshaft consists of a single cylin-drical shaft and webs, connected by means of a shrink fit. At the end of the crank-shaft is the thrust bearing flange which feeds the propeller thrust into the ship structure through thrust pads fitted to the engine casing. A characteristic of all low-speed two-stroke large diesel engines is the rigid coupling of the engine crankshaft to the ship’s propeller via the propeller shaft [3].

Because of the high levels of stress antici-pated, the main bearings are aluminium shell bearings instead of the white metal bearings used in other Wärtsilä two-stroke engines. Lubrication involves a pipe con-nection through the bearing cover, while the upper and lower connecting rod bear-ings are lubricated through the crosshead. On these two-stroke engines, the oil is fed

Dipl.-ing. patrick Friggeis Director, Two-Stroke Engine

Programs and Technologies, Research and Development

at Wärtsilä in Winterthur (Switzerland).

Dipl.-ing. saMuel aFFolteris Manager, Power System

Components, Two-Stroke Engine Programs and Technologies

at Wärtsilä in Winterthur (Switzerland).

Dr.-ing. Daniel bachMannis Manager, Structural Analysis,

Research and Common Technologies at Wärtsilä in Winterthur

(Switzerland).

Dipl.-ing. ronalD De jongis Senior Project Manager RT-Flex35/

RT-Flex40, Two-Stroke Engine Programs and Technologies,

Research and Development at Wärtsilä in Winterthur (Switzerland).

AUTHORS

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to the crosshead either using a telescopic pipe or using a knee lever, which is the solution favoured by Wärtsilä. The knee lever solution for the engines was selected following a design comparison in conjunc-tion with development partner MHI, ❹. In order to optimize the tribological conditions on the crosshead, AVL List GmbH was en -listed to carry out elasto-hydrodynamic (EHD) simulations to help with the designs of the guide rails and guide shoes.

cast cylinDer block anD coMbustion chaMber coMponents

The partition between the engine frame and the cylinder block is important for two reasons: On the one hand, it forms a safe partition between the components carrying scavenging air and exhaust gases and the crankshaft, so the power unit does not come into contact with the combus-tion products – one of the advantages of the crosshead design. On the other hand, it provides a transition between mainly welded and cast engine components. De -pending on the engine size, around 40 % of the mass of a low-speed two-stroke large diesel engine is made up of welded struc-tures. The engines utilize the longitudinal scavenging principle which is established for long-stroke two-stroke engines. There are scavenging ports at around bottom dead centre and by the central hydraulically actuated outlet valve, although the latter also has fully variable electronic controls.

The most striking innovation relative to the larger Wärtsilä two-stroke engines relates to the oil cooling for the pistons. Thanks to the thinner component wall thicknesses in absolute terms and the in -creased surface to volume ratio in the smaller combustion chamber, the system can move away from the more complex injection oil cooling using the jet shaker principle and achieve adequate component cooling by means of a simple through-flow system which is more economical to pro-duce. The geometry of the oil-bearing parts has been designed using intense Computa-tional Fluid Dynamics (CFD) analyzes such that the conventional production tolerances can be applied to guarantee suf ficient heat transfer to the coolant me dium. Here, too, the cooperation with MHI proved fruitful.

The second-largest cost factor in operat-ing a two-stroke engine – after fuel – is the

consumption of cylinder lubrication oil. A fully electronically controlled Pulse Lubri-cation System (PLS) has been utilized to achieve a low lubrication rate at the same time as a high level of reliability. This is in combination with the piston running con-cept adopted from the larger engines, which consists of plateau-honed cylinder friction surfaces with oil distribution grooves, chrome-ceramic coated, pre-profiled piston rings, gas-tight top ring and chrome-coated piston ring grooves. There is a new piston shirt with nitro-carburised friction surface to replace the otherwise standard version with set-in bronze bandages. Like the pis-ton cooling system, the cylinder collar used around top dead centre for bore cooling in larger engines is not required, as the con-

ventional coolant water sleeve is already capable of achieving the friction surface temperature profile specified.

Depending on the engine load, the run-ning-in condition and sulphur content in the fuel, all these measures add up to a cylinder lubrication rate of just 0.7 g/kWh and the aim is to gradually reduce this further as of when the engine is launched on the market.

turbocharger anD scavenging systeM

One of the key features of the new tur-bocharger and scavenging system is the ompact design with integrated auxiliary blowers, ❺. Instead of the conventional

100 %85 %

80 %

85 %

100 %

Engine speed [%]

Eng

ine

pow

er [

%]

R1

R2

R3

R4

Engine type RT-flex35 RT-flex40

Construction, cylinder number

Inline with weldedstructure, five toeight cylinders

Bore/stroke [mm]Stroke to bore ratio

Power/cylinder [kW]

Engine speed R1 [rpm]Brake mean effectivepressure [bar]

Mean pistonspeed [m/s]

Brake specific fuelconsumption, standardtuning R1 [g/kWh]Power rangeR1-R2 [kW]Engine weight(six-cylinder version) [t]

Out

put

[kW

]

80,000

60,000

50,000

40,000

30,000

20,000

10,000

8000

6000

4000

60 70 80 90 100 120 140

Engine speed [rpm]

160 180

3000

350/1550 400/17704.43

870 1135

167 146

21

8.6

176 175

3475 –6960

4550 –9080

80 125

RT-flex96CRTA96C

RT-flex82TRTA82T

RT-flex82CRTA82C

RT-flex82T-B

RT-flex84T-DRTA84T-D

RT-flex68-DRTA68-D

X72

RT-flex60C-B

RT-flex50-DRT-flex50-B

X62

RT-flex48T-DRTA48T-D RT-flex40

RT-flex35

RT-flex58T-DRTA58T-D

RT-flex58T-E

❶ Wärtsilä low-speed engine portfolio

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lateral position of the turbocharger unit, the exhaust turbocharger is placed above the output at the end of the engine, in a similar position to medium-speed four-stroke engines. The benefit of this is the reduced overall width of the unit, which means the engine can be fitted much further back in the hull of the ship as the width begins to taper.

With constant pressure turbocharging, the size of the components carrying the scavenging air is very important. The CFD flow calculations were used to optimize receiver volumes and guarantee that all cyl-inders are impacted evenly. The engines also needed to be as modular in design as possible, and this requirement has also been met: the whole range from 5RT-flex35 to

wärtsilä 14rt-Flex96c

wärtsilä 5rt-Flex35

wärtsilä 8rt-Flex40

wärtsilä 8l46F

Diesel cv Diesel pc

working process – Two-stroke Two-stroke Two-stroke Four-stroke Four-stroke Four-stroke

bore mm 960 350 400 460 132 84

stroke mm 2500 1550 1770 580 145 90

cylinDer DisplaceMent

dm3 1810 149 222 96 1.98 0.50

total DisplaceMent dm3 25,334 746 1779 771 11.9 2.00

cylinDer nuMber – 14 5 8 8 6 4

weight kg 2,300,000 69,000 153,000 124,000 995 158

power output kW 80,080 4350 9080 9600 390 110

MaxiMuM torque knm 7497 249 594 153 2.130 0.330

power / cylinDer kW 5720 870 1135 1200 65 27.5

noMinal speeD rpm 102 167 146 600 2100 4000

MaxiMuM brake Mean eFFective pressure

bar 18.6 21 21 25 22.5 20.8

power Density kW/dm3 3.2 5.8 5.1 12.4 32.8 55.1

Mean piston speeD

m/s 8.5 8.6 8.6 11.6 10.2 12

speciFic torque nm/dm3 296 334 334 198 179 165

Mean power / piston area

kW/m2 7902 9043 9032 7221 4750 4962

brake speciFic Fuel consuMption

g/kWh 171 176 175 171 217 238

Fuel consuMption at noMinal power

kg/h 13,694 766 1589 1642 85 26

Full loaD injection quantity

g/cycle 159.8 15.3 22.7 11.4 0.2 0.05

injection valves per cylinDer

– 3 x 5-hole 2 x 5-hole 2 x 5-hole 1 x 10-hole 1 x 6 … 8-hole 1 x 5 … 8-hole

average nozzle hole DiaMeter

mm 1.34 0.57 0.7 0.81 ~ 0.22 (6) ~ 0.12 (8)

speciFic weight kg/kW 28.7 15.9 16.9 12.9 2.6 1.4

❷ Technical main data in comparison with other engines

❸ Basic design six-cylinder engine

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8RT-flex40 is covered by two turbocharg er sizes (with different performance levels) and two basic designs. There is a choice of exhaust turbochargers available from ABB (A series) and MHI-MET (MB series). The aft end turbocharger system Wärtsilä has used for the first time on the new en gines has now been adapted for the RT-flex50 and is available as an option instead of the con-ventional, lateral turbocharger arrangement.

electronic engine control

At Wärtsilä, all electronically controlled two-stroke engines are called RT-flex. The system was developed entirely in-house and was introduced as a new concept in the low-speed two-stroke engines market in 2001 [4]. In line with the Wärtsilä develop-ment strategy, development work since 2008 has focussed exclusively on electroni-

cally controlled two-stroke engines. The fact that the improved flexibility, consump-tion and emissions are adapted to the per-formance figures represents real enhance-ment to customer benefits which translates into more economical fuel consumption in the medium and low-load range, ❻. Around 90 % of the engines in the current order book are electronically controlled.

The electronic engine control incorpo-rates the following component systems: : common rail injection for operation

with heavy fuel oil : electro-hydraulically actuated exhaust

valve control (i. e. without engine camshaft)

: pulse cylinder lubrication system (PLS) : engine start system with starting air

distribution.These are all based on the Wärtsilä UNIC electronic engine controller, the sensors

(e. g. crank angle detection) on which are utilized by the other component systems. The current C3 version of UNIC has been used in Wärtsilä’s common rail four-stroke engines since 2008 and is now being used in the two-stroke sector for the first time. The system is modular in design and in -corporates not only alarm, monitoring and safety functions but also selective cylinder control functions including cylinder pres-sure-based combustion control options.

coMMon rail injection systeM

Even before the design phase of the engine development process, Wärtsilä had consid ered using an established four-stroke common rail (CR) injection system on a two-stroke engine. The new smaller bore en gines to be developed met the requirements:

❹ Powertrain components

❺ Turbocharging and scavenging system

❻ Tier II tuning variants for constant nOx cycle results

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: similar sizes and compatible component dimensions

: cost-saving volume effects thanks to increased sales volumes anticipated in the small two-stroke engine sector

: benefits of time-controlled CR system compared to the previous volume- controlled CR for use on smaller, higher revving engines (dynamics)

: suitability of the smaller engines for test purposes for the basic development of an IMO Tier III-compliant system at a further stage.

A further benefit was the global research and development organisational structure within Wärtsilä with existing interdisciplin-ary project teams in place as of the begin-ning of the development phase. Wärtsilä therefore decided to develop the new injection system in conjunction with the project partners. All the project targets defined have been achieved thus far.

❼ shows a comparison of the CR injec-tion systems for Wärtsilä two-stroke en -gines with large and small bore sizes. The system for large bore sizes shown on the left-hand side of the diagram uses a servo-

hydraulically operated control unit, known as the Injection Control Unit (ICU) in con-junction with conventional injectors. An integrated volume piston provides accu-rate, crank angle resolved information for comparing the volume specifications to the actual amounts (volume control). On the right is the new system for small bore sizes, where the ICU functionality has been replaced with an electrical solenoid valve in the upper injector portion, allow-ing the fuel itself to replace the servo oil as a control medium. Injection is triggered when the built-in magnetic valve is actu-ated in the normal time-controlled way.

As a result of the changes to the system architecture, the costs incurred by the li -censee for all the injection equipment can be reduced by half relative to the same engine performance. What is all the more remarkable is the fact that the system costs for the new CR injection system are below those for a conventional me -chanical two-stroke diesel injection system for the first time.

❽ shows the supply unit on the driving end of the engine, with two high-pressure

fuel pumps and two servo oil pumps. In order to reduce the power required, the high-pressure pumps are fitted with a throttle device on the inlet side. The Wärt-silä two-stroke diesel engines currently work with injection pressures of 900 bar and 1000 bar. On both of the above sys-tems, the injection rate can be shaped by actu ating either two or three of the injec-tors placed on the circumference of the injectors in sequence. This technology is used in Wärtsilä RT-flex engines to reduce smoke and NO

x emissions in the partial load range and to ensure the engine runs smoothly even in single digit rev num-bers. The new “small” system also has the option of multiple injection for each injec-tor. In principle, it is compatible with up to 1600 bar and thus offers additional potential for developing the combustion system further.

❾ shows the individual components of the high-pressure system, with rail, pres-sure control valve, flow limiter valves and high-pressure lines developed in Effretikon, Switzerland, in conjunction with Nova Werke. The high-pressure pump and in -

❼ Common rail fuel injection systems with volumetric control for large bore engines (left) and with time control for small bore engines (right)

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jectors were developed by L’Orange GmbH, Stuttgart, Wärtsilä’s main cooperation partner in the injection component project.

cylinDer lubrication systeM

Separating the powertrain and gas ex -change areas of the crosshead engine means that the lubrication system is differ-ent on two-stroke large diesel engines than

on trunk piston engines. The cylinder lubrication oil is not connected to the sys-tem oil in the crankcase by means of an oil circuit. At the end of the day, this means a loss or continual top-up of the lubricant used up on the friction surface of the cylin-der, and this needs optimization. An exter-nal pump delivers the lubricant oil quanti-ties to each engine cylinder liner from the outside through dedicated bore holes.

The electronically controlled PLS pulse lubrication system was developed in co -operation with SKF Lubrication Systems Germany AG, Hockenheim (previously Vogel AG) and introduced for Wärtsilä two-stroke engines in 2006. The higher speed (167 rpm) of the 35 cm sets more exacting requirements of the dosing ac -curacy, frequency and minimal volumes needed. In order to comply with these,

❾ System components of the new common rail fuel injection for small cylinder bore sizes

❽ Supply unit

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the new double-acting pump CLU5 was developed. As the double-action principle only requires one action of the 4/2-way valve per pumping stroke, the overall number of cycles is halved relative to sin-gle-action pumps. ❿ shows the develop-ment of cylinder lubrication rates on Wärtsilä engines since the introduction of the Tribo-Pack standard described in [5].

DevelopMent MethoD

The so called design-for-x target-criteria-based development method has been successfully introduced and refined for a previous Wärtsilä engine product. At the beginning of the development project, the design-to-cost discipline was firmly anchored into the organisation of project and line structures at a relevant level in order to ensure that the factor of engine costs was considered and to guarantee the competitiveness of the new two-stroke engine. In practice, this resulted in clear specifications for each engine design group which represented an important, accepted control instrument both at the design phase and throughout the entire term of the project. This meant that the original costs target for the entire engine was reached just half way through the project in August 2009, at which point the target was given a moderate increase.

siMulation anD valiDation systeM

During the development phases for the new engines, major advances were also made in terms of simulation. Fatigue and

vibration analyzes were more often based on complete engine models to allow pro-jected statements on vibration behaviour of critical engine components. This was backed up by experience with earlier comparisons of calculations and measure-ments on partial models. The extensive simulation ac tivities on the first six-cylin-der version of the engine included: : fatigue assessment based on

dynamic stress calculations : multi body simulation of the

complete powertrain : elasto-hydrodynamic simulation

of plain bearings : thermo-mechanical simulation

of the combustion chamber : computational fluid dynamics of

combustion, lubrication and gas flows : harmonic response simulation of

the entire engine vibration regime : explicit dynamic FE simulation of

high velocity impacts on fuel injection components.

The expectation is that the accuracy of simulation results will increase signifi-cantly after the measurement campaigns planned for the first engine have been analyzed and that this will be beneficial to internal know-how and future engine development.

This is why there are plans to carry out a 500-h comprehensive engine testing programme in cooperation with licensee 3. Maj Engines & Cranes in Rijeka, Croatia, which will be a first of its kind when it comes to the testing of two-stroke engines. At the same time, the extended perfor mance tests will help validate the newly developed component systems and

how they interact in the engine. Essen-tially, the functionality of newly devel-oped Wärtsilä components and systems is guaranteed by testing on specially built functional and continuous test beds for a period >3000 h. At the Wärtsilä Diesel Technology Centre in Oberwinterthur, functional and durability test rigs were purpose-built for the fuel injection and PLS cylinder lubrication systems of the new engine.

suMMary anD outlook

The Wärtsilä RT-flex two-stroke 35 cm and 40 cm bore marine diesel engines are low-cost, high-tech products whose develop-ment has focused more than ever on excellent reliability and low life-cycle costs for the customer. Development work involving new, sophisticated methods has been successfully completed. The first RT-flex35 six-cylinder engines are currently under construction. Tests are due to finish in November 2011.

reFerences[1] Heim, K.; Frigge, P.: The Wärtsilä low-speed engine programme for today’s and future require-ments. 26th CIMAC World Congress 2010, Bergen, norway[2] Sakabe, H.; Hosokawa, n.: Cutting edge technologies of UE engine for higher efficiency and environment. 26th CIMAC World Congress 2010, Bergen, norway[3] Mollenhauer, K.; Tschöke, H. (Pub.): Hand-buch Dieselmotoren. 3rd Edition Berlin: Springer, 2007[4] Demmerle, R.; Heim, K.: Evolution of the Sulzer RT-flex common rail system. 24th CIMAC World Congress 2004, Kyoto, Japan[5] Amoser, M.: Verbesserter Kolbenlauf von langsam laufenden Dieselmotoren. In: MTZ 63 (2002), no. 4

❿ Lubrication oil feed rate development for Wärtsilä two-stroke engines

The authors thank Alexander Bühner,

Ole Christensen, Dr. Wilfried Schiffer and Mar-

tin Sichler for their support in the writing of

this paper.

THANKS

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